Ukrainian air force pilots march in their air base in Belbek. Photo: AFP

Some internet and telephone services were severed after Russian forces seized control of airfields and key installations in Ukraine's Crimea region on Friday, but now government officials were being targeted, said Valentyn Nalivaichenko, the head of Ukraine's SBU security service.

"I confirm that an ... attack is under way on mobile phones of members of Ukrainian parliament for the second day in row," the security chief told a news briefing.

"At the entrance to [telco] Ukrtelecom in Crimea, illegally and in violation of all commercial contracts, was installed equipment that blocks my phone as well as the phones of other deputies, regardless of their political affiliation," he said.

Ukrtelecom has already said armed men raided its facilities in Crimea on Friday and tampered with fibre-optic cables, causing outages of local telephone and internet systems on the continent.

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The Ukrainian security chief did not say whether the new issues were linked to the earlier raid or a separate tampering incident. Ukrtelecom said it was working on a response to questions about Nalivaichenko's remarks.

Russia's domestic intelligence service, the FSB, declined to comment when asked if Moscow was behind the communications disruptions in Ukraine.

The main Ukrainian government website, kmu.gov.ua, was offline for about 72 hours after Russian forces seized control of the peninsula, but went back up early on Monday, said John Bumgarner, chief technology officer for the US Cyber Consequences Unit.

Bumgarner, whose firm advises companies and government agencies on how to fend off cyber attacks, said he is not sure the site went down as a result of a cyber attack. Still, he said he believes Moscow has the ability to cause major disruptions using cyber operations.

"I know they have the ability to do at least as much damage as they did in Estonia and Georgia," he said.

Estonia suffered a 10-day attack on its internet services in 2007, which caused major disruptions to its financial system, during a spat with Moscow over a Soviet-era war memorial, and Georgia was hit by mass cyber attacks during a brief 2008 war with Russia over its pro-Moscow South Ossetia region.

Russian authorities denied direct involvement in both attacks, saying they had no influence over the actions of self-styled patriotic hackers.

Russia holding back?

Much of Ukraine's telecommunications infrastructure was built when it was part of the Soviet Union, along with what is now the Russian Federation, and is particularly vulnerable to penetration by Moscow.

"The Russians have the place completely wired," said Jim Lewis, a former US foreign service officer and now senior fellow at the Centre for Strategic and International Studies in Washington.

"They are right next door and most traffic has to go through Russia. That they haven't done more probably reflects their confidence that they're going to come out ahead and there's nothing anyone can do about it," Lewis said.

Cyber warfare experts say that while Russia certainly has the ability to conduct such campaigns against Ukraine, it has yet to need to use those capabilities.

"This would show the Russians acting with more discretion and targeting than recently," said John Bassett, former head of the London and Washington stations of GCHQ, Britain's top-secret government communications centre.

"This wouldn't expose any great depth of their technological capability and they would be keeping the harder stuff back," said Bassett, now associate at Oxford University's Cyber Security Centre.

Marty Martin, a former senior operations officer with the CIA, said Moscow would likely only take action to damage Ukraine's internet and internal communications systems if hostilities broke out.

"A lot of times you don't want to shut things down. If you do that, then you don't get your flow of intelligence. You are probably better off monitoring it," Martin said.

Experts believe Russia was behind the hacking of a confidential phone conversation between senior US State Department official Victoria Nuland and US Ambassador to Ukraine Geoffrey Pyatt, which was leaked over YouTube last month.

"Russia's strategy is control the narrative, discredit opponents, and coerce," Lewis said.